Error-free transmission of information over a communication link, and in particular over a wireless communication channel, remains a fundamental challenge for communications engineers. Various techniques have been employed in the physical layer to make the communications link more robust, such as advanced modulation schemes, channel coding, and the like. Likewise, error checking and error correction techniques have been deployed in the data link layer to improve the link's reliability. However, the reliability of the communication channel remains important because of the continuing demand for more mobility and improved reliability, coupled with seamless handovers and higher data rates.
Various methods such as Automatic Repeat Request (ARQ), code combining, incremental redundancy, and Chase combining are commonly employed to reduce a link's bit-error rate (BER) and frame-error rate (FER). However, each of these techniques has both advantages and disadvantages and some are not so effective under demanding conditions.
For example, conventional error correcting codes are most effective when the errors are randomly distributed throughout the bit stream, but are less effective when a stream of bits includes several errors grouped closely together. For this reason, an interleaving procedure is used in many wireless systems, to exploit time diversity and reduce errors in transmission by distributing the errors across several separately encoded groups of bits so that error correcting codes are more effective. For instance, in GSM systems for SACCH control channel transmission, the SACCH bursts occur once every 26 bursts or after a time separation of 26*TDMA frame duration=26*4.615 mSec =120 mSec. (See 3GPP TS05.03, §4.14.) Each SACCH block's data (before encoding 184 bits, and after encoding 456 bits) is divided, with an interleaver, into four SACCH bursts for transmission. So, consecutive bits are time separated by (4*26*4.615) =480 mSec.−this clearly offers time diversity. But, this is still not enough to assure error-free transmission.
The ability of an interleaving process to improve the performance of an error-correcting code is directly related to the “coherence time” of the transmission channel. A coherence time of Tc for a given channel indicates that if two frames are transmitted at an interval of Tc, then there is a good probability that the channel has changed in the meantime (e.g., channel state has been changed from good to bad or vice versa), so that the frames experience different channel states.
The coherence time of a wireless channel depends, among other things, on the relative movement between the transmitter and the receiver, which is often represented as:
                                          T            c                    =                      1                          4              ⁢                              D                s                                                    ,                            (        1        )            where Ds is the Doppler spread induced by movement of a mobile station relative to the base station. Since Ds=2fv/c, where f is the transmission frequency, v is the relative speed between the transmitter and receiver, and c is the speed of light, a typical coherence time for a GSM channel might be:Tc=c/(8fv)≈3*108/(8*900*106*(30*103/3600))≈5 ms,  (2)given a center frequency of 900 MHz and a relative movement between transmitter and receiver at 30 kilometers per hour. This implies that if a channel is in deep fade at a given time, then after a 5 ms time interval there is significant probability that the same channel will not be in the same fading state. So, if the same data is sent twice with a time gap between transmissions of more than 5 ms, then it is unlikely that both transmissions will suffer the same fading effect. Thus, at least one of the transmissions might experience relatively good channel conditions.
In many systems, this time-diversity approach is exploited through use of a Chase combining technique to improve the signal quality of certain. Received information corresponding to an erroneously received block is kept and combined with information corresponding to the retransmission of the block. With this algorithm, decoding performance is improved because the two blocks carry the same information but are corrupted differently by the channel.
In GSM systems, the Slow Associated Control Channel (SACCH) and the Fast Associated Control Channel (FACCH) carry important system information and require some extra amount of protection in order keep the wireless link active and robust. Reliability of these control channels is particularly important because call-drop and handover issues most often arise because of poor signal reception in an area of weak wireless signal coverage area or because of short-term fading effects, either of which can cause control channel decoding to fail. The effect of these short duration channel corruptions is not quite as severe for voice signals, since traffic channel decoding errors often result in a situation where no speech is heard for a short time but resumes when the channel improves. If control channel decoding remains reliable, the link can usually be maintained through short-term impairments.
As a result, it is very useful to have even small performance gains for decoding of SACCH or FACCH logical channels. To provide some extra protection for these control channels against temporary severe channel degradations, 3GPP has introduced a feature called Downlink Repeated FACCH and Repeated SACCH. The Repeated SACCH feature was first proposed in 2005 and was standardized to become part of Release 6 of the 3GPP specification in 2005.
The Repeated SACCH procedure is described in a specification document adopted by the 3rd-Generation Partnership Project (3GPP): 3GPP TS 44.006, “Mobile Station-Base Station System (MS-BSS) interface; Data Link (DL) layer specification,” v. 10.0.0, March 2011. According to the details of this specification, for SACCH reception on the downlink a mobile station first attempts to decode a SACCH frame by itself. If this decoding fails, as indicated by a failed Cyclic Redundancy Check (CRC), the mobile notifies the network by setting the SACCH Repetition Request indicator bit on the uplink SACCH to “SACCH Repetition Required”. Next, when the mobile receives the repeated SACCH block, it makes a second attempt to decode the frame, this time after combining it with the previously received SACCH frame.
In the repeated transmission, the same information (frame) is transmitted the second time, without any modification or any other change in the retransmitted burst. After both bursts of data have been received, their energy values are combined and fed to a decoder to make a hard decision. Thus, the energy corresponding to each bit in the frame is effectively averaged, via the combining, and then compared to a threshold value to determine whether the received bit is high or low.
The Repeated SACCH technique only partially exploits time diversity, and there is further scope left to improve this technique in the receiver implementation technique. Because repeated SACCH and FACCH is generally utilized only during conditions of low signal power, the SACCH and FACCH message blocks suffer from both AWGN noise and channel fading. Accordingly, further improvements in decoding these messages are needed.